I have a strange question for you all.

I’ll be in Jerusalem in March and plan to do a lot a photography. One place I’ll know I’ll go is a 2700 year old water tunnel and I’d like to get some pictures from inside. Hezekiah's Tunnel is mostly small – 3 feet wide, 5-6 feet high. The water in it ranges from knee deep to waist deep.

Here are some pictures:
http://www.bibleplaces.com/heztunnel.htm

The question I have is how to shoot in the tunnel. I have a 300d and a Promaster 7500 flash. Tripod is not practical and there is no natural light (pitch dark). My fear is that the camera won’t have any idea how to meter this and the close rock wall will be terribly overexposed.

Any suggestions of how to approach this? Thanks!